Optical detection of water droplets using light refraction with a mask to prevent detection of unrefracted light

ABSTRACT

An optical detection system for detecting rain or other water droplets on the outer surface of a window and fog on the inner surface with a single photo-detector. The invention measures the accumulation of water droplets on the window by light refraction of a first light beam with droplets to redirect a first light beam to the photo-detector. A masking device prevents light from reaching the photo-detector directly without refraction. As a result the rain measurement output signal of the photo-detector increases with an increasing accumulation of water droplets on the window. The fog accumulation is measured by a second light beam reflected off the inner surface of the window to the photo-detector so that the output signal of the photo-detector decreases with increasing amounts of fog since fog scatters the light to reduce the amount of light reflected to the photo-detector. A third light source is focused directly on the photo-detector to bias it into an operating point of high sensitivity to infrared light and is connected in a negative feedback circuit from the output of the photo-detector amplifier. The photo-detector output is connected to a narrowband amplifier that is tuned to the frequency of an oscillator which pulses the first and second light sources at different times. As a result of this negative feedback, changes in the output signal due to external factors are cancelled so they do not produce errors.

The subject matter of the present invention relates generally to opticaldetection of water and in particular to optical detection of waterdroplets on a window, such as the windshield of an automobile or othervehicle using light refraction. A light beam is transmitted through thewater droplets which refract the light beam to a photo-detector whichproduces an electrical measurement signal at the output of thephoto-detector corresponding to the amount of water accumulation on thewindow. The apparatus and method of the present invention is especiallyuseful for detecting moisture, including rain drops and fog or otherprecipitation on the windshield of automobiles or other vehicles inorder to operate windshield wipers, heaters, fans and other devices forremoving such rain and fog to improve the visibility through suchwindshield or for operating other devices such as motors for closingconvertible tops, sunroofs, or other windows in the event of rain whenthe vehicle is left unattended.

BACKGROUND OF THE INVENTION

It has previously been proposed in U.S. Pat. No. 4,131,834 toBlaszkowski, issued Dec. 26, 1978, to provide moisture detectors basedupon measuring changes in electrical conductivity between spacedelectrodes which sense rain when the gap between such electrodes isbridged by the rainwater. However, the amount of conductivity varieswith atmospheric contaminants in the water as well as corrosion and wearof the electrical contacts forming the electrodes. Therefore suchmoisture detectors do not provide accurate measurement of the amount ofmoisture present.

In addition, moisture detectors have been proposed for detectingmoisture based on measuring the changing capacitance in the gap betweenspaced electrodes due to changes in the dielectric material of such gap,such as when water is present. However, such a moisture detector suffersfrom poor sensitivity due to the proximity effects of moving wiperblades on such capacitance and from interfering electrical fields frompower lines and other sources.

It has also been proposed to detect moisture by sensing the soundcreated by infringing droplets but this is inaccurate and is unable todetect light mists or fog accumulations. Similarly, moisture detectorsbased upon measurement of the mass changes due to the presence of waterdroplets are insensitive to light mist or fog.

Some optical detectors have sensed moisture based upon the interruptionof light beam by the water droplets. However, these detectors also areinsensitive to gradual accumulations of moisture as mist or fog. Also,windshield wipers interrupt the light beam and require gating mechanismsto disable the light detector during wiper sweeps so they are somewhatimpractical.

It is believed that moisture detectors which sense water droplets bylight refraction within the droplets are a substantial improvement overthese moisture detectors. However, previously optical detectors whichdetect raindrops based upon light refraction have suffered from severaldisadvantages, including small detecting area, low sensitivity, errorsignals due to ambient light, and dependence upon long-term stability oflight sources and photo-detectors whose characteristics changesignificantly with temperature, aging, operating point, and supplyvoltage variations.

The optical detection method and apparatus of the present inventionovercomes these problems using a first light beam transmitted through alarge area of the window and by employing a mask which prevents thefirst light beam from directly reaching the photo-detector unless suchlight beam is refracted by water droplets on the outer surface of thewindshield. As a result, the output signal of the photo-detectorindicating the presence of water droplets is zero when no droplets arepresent and increases in amplitude with the size and amount of waterdroplets present on the windshield of the vehicle or other window for amore accurate and more sensitive measurement of the accumulation of rainon such window.

A second light source may be provided for measuring fog by reflecting asecond light beam off the inner surface of the window to thephoto-detector in order to detect fog on such inner surface. As a resultof diffusion of the second light beam by the fog less light is reflectedoff of the window to the photo-detector so that the fog measurementsignal decreases in amplitude with increasing amounts of fog. The outputsignal of the photo-detector for measuring the accumulation of fog isdistingnished from that for measuring the accumulation of rain byoperating the two light sources at different times such as byelectronically switching the inputs of two current amplifiers drivingsuch light sources in an alternating manner to the output of a singleoscillator. A third light source directly radiates light upon thephoto-detector to bias it to the proper operating point. A narrowbandamplifier tuned to the oscillator frequency is connected to the outputof the photo-detector transistor to amplify the rain and fog measurementsignals. The output of such amplifier is connected through a negativefeedback circuit to the third light source to cancel gain changesproduced by changes in ambient light, temperature changes and aging ofthe light source and photo-transistor, and power supply variations.

It has been previously proposed in U.S. Pat. No. 5,059,877 to Teder,issued Oct. 22, 1991, to operate a windshield wiper on an automobileautomatically by the optical detection of water droplets on thewindshield using light reflection from the outer surface of thewindshield. An accumulation of raindrops on such outer surface scattersor diffuses the light beam and reduces the output signal of thephoto-detector with increases in raindrop accumulation. Thephoto-detector is a photo-transistor which is coupled to the windshieldby a light pipe of small diameter which greatly reduces the measuredarea or the windshield to less than approximately 1 sq. cm. This reducesthe sensitivity of measurement, especially to a small accumulation ofraindrops. The optical detector system of the present invention solvesthese problems by using light refraction with a masking device in frontof the photo-detector and a wider light beam which covers a much largerarea of the windshield, over 31 sq. cm. This larger measurement areagreatly improves the accuracy of measurement of the amount ofaccumulated rainfall. Also, the present invention operates in a moreefficient manner by refracting the light beam with the water droplets toredirect it toward the photo-detector which is shielded from directradiation of such light beam by the masking device. As a result theoutput signal of the photo-detector increases with an increase in theamount of raindrops thereby improving its sensitivity. In addition, theTeder rain measurement system is more sensitive to changes in ambientlight levels and therefore requires that a compensation circuit sampleand store the ambient light level signals for subtraction from themeasurement signal. Also, high ambient light levels including brightsunlight or at night when the headlights of an approaching car strikethe windshield at a light intensity greater than predetermined limitscause the raindrop detection and wiper operation process to be suspendedtemporarily. This ambient light problem is avoided in the opticaldetector of the present invention by employing oscillator pulsed lightsources, a narrowband amplifier at the output of the photo-detectortuned to the oscillator frequency and negative feedback from the outputof such amplifier through a bias light source directed at thephoto-detector.

U.S. Pat. No. 4,867,561 to Fujii et at., issued Sep. 19, 1989, alsoshows a similar optical detector for detecting rain by light reflectionfrom the windshield in a detection area of extremely small size of lessthan 2 sq. cm. The photo-detector is two-dimensional array ofphotoelectric transducer elements mounted within an optical systemhousing supported beneath the dashboard closely adjacent the windshield.This optical detector employs light reflection for sensing raindrops onthe outer surface of the windshield so that the presence of theraindrops reduces the amount of light which is reflected to thephoto-detector and thereby reduces the output signal of suchphoto-detector. As a result the Fujii detector system has limitedsensitivity and reduced accuracy compared to that of the presentinvention. Ambient light level changes are also a problem with thisdetector. Thus the ambient light level is measured and used to reducethe threshold levels of the comparators in the detection circuit formeasuring rain and fog in an attempt to reduce inaccuracies due tochange in the ambient light level. Also no measurements may be made ifexcessive ambient light is present such as bright sunlight.

A similar teaching is also shown in U.S. Pat. No. 4,595,866 to Fukatsuet at., issued Jun. 17, 1986, which relates to an optical detector fordetecting rain on the windshield by the transmission of light from anexternal light source outside the windshield to a photo-detector withinthe automobile. The light beam is transmitted directly to thephoto-detector, so that the output signal of the photo-detector isreduced when raindrops accumulate on the outer surface of the windshieldbecause they refract the light beam away from such photo-detector. Thepresent invention differs by providing a mask in front of thephoto-detector to prevent light from being transmitted directly from thelight source to the photo-detector and refracting a portion of the lightbeam with the detected raindrops to the photo-detector. As a result theoutput signal of the photo-detector increases with increasing amounts ofraindrops on the windshield. The light detector of Fukatsu et al.consists of a plurality of pairs of photo-detectors, each photo-detectorof a pair being positioned behind either an infrared transparent stripor an infrared opaque strip with the outputs of said pair ofphoto-detectors being connected to a differential amplifier to measurethe amount of rain accumulating on the windshield. This optical detectoris more complicated, expensive and bulky. Also, it suffers from theproblem of ambient light because changes in ambient light would effectthe output signals of both photo-detectors of each pair. Finally, thereis no way of differentiating from the light detection of raindrops onfie outside surface of the windshield and the detection of fog on theinner surface of the windshield.

The optical detection method and apparatus of the present invention hasseveral advantages over the above-discussed prior art, including theability to monitor a much larger area of rainfall on the windshield sothat the output signal of the photo-detector is more accurate inmeasuring small accumulations of randomly located droplets. In addition,by employing a mask to block light from being directly transmitted fromthe light source to the photo-detector and by employing light refractionfrom the raindrops to redirect the light to the photo-detector, theoutput signal of the photo-detector increases with increasing amounts ofrain to provide more sensitive detection at the onset of rain. Also, thephoto detection method and apparatus of the present invention is capableof detecting small amounts of rain in the presence of high ambient lightand is not effected by changes in ambient light. The optical detectionmethod and apparatus of the present invention also eliminates errors inthe photo-detector output signal due to external factors unrelated tomoisture, such as changes in temperature and aging of the LED lightsources and photo-detector, power supply voltage variations or changesin ambient light by employing negative feedback through a referencelight source. This reference light source sets the bias of thephoto-detector to an operating point of high sensitivity to infraredlight, and cancels any changes in the photo-detector output signal dueto these external factors by negative feedback from the output of atuned amplifier connected to the photo-detector transistor through afeedback circuit to the reference light source.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide animproved moisture detection method and apparatus of high accuracy andsensitivity in which water droplets on a window are detected by opticaldetection using light refraction in the droplets.

Another object of the present invention is to provide such a moisturedetection method and apparatus of high sensitivity in which a lightmasking device is positioned in front of a photo-detector to prevent itsdirect irradiation by a light beam which is refracted by such waterdroplets to the light detector to produce a measurement output signalthat increases in amplitude with increasing amounts of moisture tomeasure the amount of moisture accumulation on the window.

A further object of the invention is to provide such a moisturedetection method and apparatus using light refraction of a first lightbeam for measuring the presence of rain or other water droplets on theouter surface of a window and which employs a second light beam forreflecting light off the inner surface of such window to the samephoto-detector in order to detect fog on such inner surface anddistinguishes between rain and fog measurements by selectively switchingbetween such first and second light beams.

An additional object of the present invention is to provide such animproved moisture detection method and apparatus in which the samephoto-detector is used to detect the first light beam and the secondlight beam for measuring raindrops and fog in an efficient and accuratemanner.

Still another object of the invention is to provide such a moisturedetection method and apparatus in which the area of the first light beamwhich strikes the window for detection of water droplets on the windowis greatly increased in size to provide a more accurate raindropaccumulation measurement signal.

A still further object of the invention is to provide such a moisturedetection method and apparatus in which a third light source is employedto provide a reference light beam for irradiating the photo-detectordirectly in order to bias the photo-detector at a proper operating pointof high sensitivity to such light and whose bias current supply circuitis connected in a negative feedback path from the output of thephoto-detector amplifier to the third light source to cancel changes inthe output signal due to external factors including temperature changesand aging of the light source or photo-detector, supply voltagevariations, and ambient light changes.

A still additional object of the invention is to provide such a moisturedetection method and apparatus in which the first and second lightsources are connected to an oscillator for pulsing such light sources toproduce a pulsed output signal of the photo-detector and for amplifyingsuch output signals with a narrowband amplifier having a tank circuitwhich is tuned to the oscillation frequency to reject other potentiallyinterfering error signals which might be produced by the photo-detector.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will be apparentfrom the following detailed description of certain preferred embodimentsthereof and from the attached drawings of which:

FIG. 1 is a diagram of one embodiment of a moisture detection systemusing the method and apparatus of the present invention suitable fordetecting water droplets on the outer surface of the window byrefraction of a first light beam with such droplets and for detectingfog on the inner surface of the window by reflection of a second lightbeam from such inner surface;

FIG. 2 is a schematic diagram of a second embodiment of the moisturedetector system of the present invention in which the mask used in FIG.1 for preventing light from being transmitted directly to thephoto-detector from the first light source used for detecting waterdroplets is changed to a horizon-type mask which blocks the lowerportion of the first light beam;

FIG. 3 is a schematic diagram of a third embodiment of the moisturedetection system of the present invention which the first light sourcefor measuring the accumulation raindrops on the outer surface of thewindshield is moved to a position inside the window and is directed sothat its light beam is reflected off of an external mirror positionedoutside of the window before striking the water droplets and beingrefracted by such droplets to the photo-detector, such internal lightsource acting as the masking device to prevent light from such firstlight source from reaching the photo-detector directly without beingrefracted;

FIG. 4 is a side view of the preferred embodiment of the moisturemeasurement apparatus of the present invention used in a measurementsystem in accordance with a modification of the system of FIG. 1;

FIG. 5 is a plan view of the moisture measurement apparatus of FIG. 4;

FIG. 6 is a block diagram of the electrical circuit used for themoisture measurement systems of FIGS. 1-4; and

FIG. 7 is an electrical circuit of a portion of the block diagram ofFIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

As shown in FIG. 1, one embodiment of the moisture measurement system ofthe present invention includes a first light source 10 positionedoutside a light transparent window 12 such as the windshield of anautomobile or other vehicle. The first light source 10 may be mountedunder or on top of the automobile hood and spaced from a photo-electricdetector 14 mounted inside of such window. A lens 16 in front of thefirst light source focuses the light into a first light beam having acentral axis 18 and a conical shape such light beam being defined by anupper ray 20 and a lower ray 22 and intersecting the window over a largearea of measurement. In this system the central axis 18 of the firstlight beam is aligned with the photo-detector 14 which may be aphoto-transistor.

The first light source 10 is preferably a light emitting diode (LED)which when energized emits a narrow beam of infrared light 24 thatpasses through the lens 16 and is focused by such lens into the firstlight beam bounded by outer light rays 20, 22. While the light beam maybe of visible light it is preferably of infrared light to avoiddistraction of the vehicle driver.

In the embodiment of FIG. 1 a light opaque masking device 26, such as ametal plate, is provided in front of photo-detector 14 and on the axis18 of the light beam to prevent the first light beam from directlyirradiating the photo-detector. Thus, the central viewing axis 28 of thedetector 14 through its lens 32 is aligned with the beam axis 18 along acommon axis and the mask 26 is positioned across this common axis sothat in absence of any water droplets upon the window 12 thephoto-detector 14 does not receive the first light beam and producessubstantially no output signal. However, when a plurality of waterdroplets 30 accumulate on the outer surface of the window 12 such waterdroplets refract the first light beam and cause a portion of it to beredirected to the photo-detector 14. As a result, an output signal isproduced by the photo-detector whose collector current amplitude is ameasurement of the amount of water droplets accumulated on the outersurface of the window. The moisture measurement value corresponds toboth the number of water droplets and the area or size of such waterdroplets within the measurement area on such window. Thus, the upperbeam limit ray 20 is refracted downward by the water droplet andredirected as refracted light ray 20A. Similarly, the lower beam limitray 22 is refracted upward by a water droplet and redirected asrefracted light ray 22A. Both of the refracted rays 20A and 22A arcfocused by a lens 32 to the photo-detector 14.

A second light source 34 is provided inside the window to detect fog orother moisture on the inner surface of the window 12. Thus, the secondlight source 34 may be another light emitting diode (LED) which emitsinfrared light to produce a second beam 36 that is focused by a lens 38on the inner surface of the window. This second light beam is normallyreflected off the inner surface of the window 12 as reflected beam 36Adirectly to the photo-detector 14 to produce a fog measurement outputsignal. When fog or other moisture is present on the inner surface ofthe window 12 a portion of the second light beam 36 is scattered anddiffused by the moisture so that such portion is no longer reflected tothe photo-detector 14. As a result, the fog measurement signal producedby the photo-detector 14 decreases in amplitude with greateraccumulations of fog on the inner surface of the windshield. It shouldbe noted that the second light source 34 is switched on at differenttimes than the first light source by an electronic switch circuit inorder-to distinguish the fog measurement signal from the rainmeasurement signal produced by the photo-detector 14 in a mannerhereafter described with respect to FIG. 6.

A third light source 40, such as an infrared LED, emits a third lightbeam 42 which acts as a reference light beam and is transmitted througha lens 44 directly to the photo-detector 14 in order to bias thephoto-detector at a preferred operating point on its characteristiccurve where it is of high sensitivity to infrared or other light emittedby sources 10 and 34. In addition, the third light source 40 may providean optical negative feedback connection for a circuit (not shown) from aphoto-detector output amplifier (not shown) to the photo-detector in amanner hereafter described with respect to the circuit of FIG. 6 inorder to eliminate any changes in the measurement output signals of suchphoto-detector due to external factors such as temperature changes,aging of the light sources or photo-transistor, variations in the powersupply voltage, and ambient light changes.

As shown in FIG. 2, a second embodiment of the moisture detection systemof the present invention differs from that of FIG. 1 by employing ahorizon-type mask 46 which blocks the lower portion of the first lightbeam 20, 22. The central viewing axis 28 of the photo-detector 14 andits associated lens 32 is not in alignment with the center axis 18 ofthe first light beam but is blocked by the mask 46 so that substantiallynone of the first light beam directly irradiates the photo-detector.However, when raindrops 30 accumulate on the outer surface of the window12 they refract the light beam so that the upper periphery ray 20 isrefracted downward as ray 20A to the photo-detector while the centralaxis ray 18 of the first light beam is also refracted down as refractedray 18A to such photo-detector thereby causing an increase in theamplitude of the rain measurement output signal of the photo-detector asa measurement of the amount of raindrop accumulation on the window. As aresult of upwardly inclining the center axis 18 of the first light beamso that it is not in alignment with the central viewing axis 28, itsbright center region is used to measure the rain droplets which refractsuch light beam and redirect the refracted beam ray 18A to thephoto-detector. This improves the sensitivity of the photo-detector todetecting raindrop accumulation. Other than these changes, the secondembodiment of FIG. 2 is similar to that of FIG. 1 and the same referencenumerals have been used in FIG. 2 to designate like parts.

As shown in FIG. 3 a third embodiment of the moisture detection systemof the present invention differs from that of FIG. 1 by positioning thefirst light source 10 and its associated lens 16 on the inside or thewindow 12 and adding an external mirror 48 positioned outside of thewindow. As a result, the first light beam 20, 22 emitted by the firstlight source is focused by lens 16 and transmitted through the window tothe mirror 48 which reflects the first light beam back through thewindow so that such beam is refracted to the photo-detector 14 throughits associated lens 32 when rain droplets 30 are present on the outersurface of the window. However, when no raindrops are present on theouter surface of window 12 the boundary rays 20A, 22B of the first lightbeam are not redirected to the photo-detector but instead are redirectedso that they do not reach the photo-detector. It should be noted that inFIG. 3 the light beam passes through the window twice and is thereforeattenuated more than that of FIG. 1 so that this system is not assensitive as FIG. 1. Also in FIG. 3 the first light source 10 functionsas a masking device in front of the photo-detector 14 thereby replacingthe mask 26 of FIG. 1 and blocking the central viewing axis 28 of thephoto-detector from directly receiving any unrefracted light from thefirst light source. The second light source 34 and the third lightsource 40 function in a similar manner in FIG. 3 to their correspondingelements in FIG. 1 and will not be described further. It should be notedthat each of the three light sources 10, 34, 40 in all of theembodiments of FIGS. 1-3 are preferably light emitting diodes (LED)which emit light of the same wavelength, preferably infrared. Also, thephoto-detector 14 is preferably a photo-transistor which is sensitive toinfrared light.

A preferred embodiment of the moisture detection apparatus of thepresent invention is shown in FIGS. 4 and 5 which provides a modifiedversion of the optical detection system shown in the schematic diagramof FIG. 1. The apparatus includes first infrared light source LED 10 andassociated lens 16, second infrared light source LED 34 and associatedlens 38, an infrared photo-detector transistor 14 and associated lens32, and third infrared light source LED 40 and associated lens 44. Thesethree light sources and the photo-detector and their lenses are allsupported ha a similar manner to FIG. 1 to be properly positioned withrespect to the window 12 which may be the windshield of an automobile orother vehicle. The first light source 10 and its lens 16 are supportedon top of or beneath a hood 50 of the automobile so that the centralaxis 18 of the first light beam is inclined at an angle of about 13° toa horizontal reference plane 52 and intersects the windshield at point53 but is not in alignment with the central viewing axis 28 of thephoto-detector 14. Instead, unlike FIG. 1 only a lower portion of thefirst beam, not the center axis 18, is blocked by the mask 26 to preventsuch lower portion of the first light beam from being directlytransmitted to the photo-detector. A construction line 55 extending fromthe center of mask 26 to the first light source is at an angle of 10°with respect to the horizontal reference plane 52 and as an angle of 3°with respect to central axis 18. As a result the central axis 18 of thefast light beam and its corresponding bright center pass above the mask26 and is refracted to the photo-transistor 14 by water droplets on theouter surface of the windshield 12 to measure the accumulation ofraindrops with greater sensitivity.

The windshield 12 forms an angle of approximately 30° with thehorizontal reference plane 52. The center viewing axis 28 of the viewingfield of the photo-transistor 14 and its associated lens 32 intersectsthe center axis 18 of the first beam on the outer surface of thewindshield 12 at point 53. Viewing axis 28 makes an angle of 4° with theconstruction line 55 through the first light source 10, such angleextending above such line. The center axis of the third light beam 42also makes an angle of about 4° with respect to the construction line55, such angle extending below such line, and intersects the center ofthe lens 32 of the photo-transistor. The second light source 34 emitsthe second light beam 36 which forms an angle of incidence of 28° withrespect to the inner surface of the windshield 12 and the reflectedsecond beam 36A forms an angle of reflection of 28° with such innersurface of the windshield as it is reflected to the photo-transistor.Also, the axis 36 of the second light beam forms an angle of 44° withrespect to the axis 42 of the second light beam.

The second light source 34 and the third light source 40 are bothmounted on a metal support plate 54 which is connected by a swivel joint56 at one end of such plate to an L-shaped support bracket 58 welded toa flat support plate 60 which may be cemented to the bottom of thewindshield. The opposite end of the support plate 54 is secured by ascrew 62 to a suitable support member 64 fixed to the upper surface ofthe dashboard of the automobile. The photo-detector 14 and itsassociated lens 32 are secured to the upper surface of the opposite endof support plate 50 within a tubular housing 66 welded to such plate andhaving an over-hanging hood which shields the photo-detector fromambient light sources. The third light source 40 is mounted within afirst tubular member 68 which extends within a second tubular member 70fixed to plate 54. The mask 26 is mounted on the top of the tubularmember 70 which is of the proper inner diameter to receive the firsttubular member 68 and to hold it in a sliding fit enable the third lightbeam of light source 40 to be transmitted therethrough to thephoto-detector. The second light source 34 is fixed by a bracket 72welded to the top of the first tubular member 68 to enable alignment ofthe center axis of the reflected second beam 36A with the photo-detector14 by pivoting the first tubular member within the second fixed tubularmember 70.

In the preferred embodiment of FIGS. 4 and 5 the first light source 10is spaced a distance of about 63/8" from the windshield 12 atintersection point 53 along its center axis 18. The photo-detector 14 isspaced a distance of about 5-7/16" along its central viewing axis 28from the windshield at the intersection point 53. The third light source40 is spaced along axis 42 a distance of 4-15/16" from the lens 32 ofphoto-detector 14. It should be noted that the cathode leads of thesecond light source 34 and the third light source 40 may each have oneterminal connected together to provide thermal coupling for temperaturecompensation.

A moisture detection measurement circuit in accordance with the presentinvention is shown in FIG. 6 and includes an oscillator 74 whichproduces a square wave output signal having a frequency of approximately37 Kilohertz. The oscillator output signal is supplied through anelectronic switch 76 having two output terminals 75 and 77 connectedrespectively to the inputs of a pair of current drive amplifiers 78 and80 which drive the first light source 10 and the second light source 34,respectively. Thus, the output signal of the oscillator 74 is applied toa selected one of the driver amplifiers 78 and 80 in accordance with theposition of the switch 76 so that one of the light sources 10 and 34 ispulsed at a time to measure rain or fog by the same photo-detector 14 atdifferent times. In addition, a bias current source 82 is connected tothe third light source 40 and such current source is connected to asource of DC bias voltage 84 which biases the third light sourcenormally on. The third light source 40 normally biases thephoto-detector transistor 14 to its proper operating point for highsensitivity to infrared light. The output of the photo-detector 14 whichmay be a photo-transistor, is connected to a tuned amplifier circuit 86which includes an RC tank circuit tuned to the 37 Kilohertz frequency ofthe oscillator 74. The sine wave output signal of the tuned amplifier 86is transmitted through an RC phase shifter circuit 87 forming part of anegative feedback circuit 88 which is connected from the output ofamplifier 86 to the bias current source 82 to amplitude modulate thethird light source with a negative feedback sine wave signal. Also, thethird light source 40 may be thermally coupled to the second lightsource 84 such as by connecting their cathode leads together for thermalcompensation. As a result of this negative feedback, any externalchanges in the photo-transistor output signals due to power supplyvariations, temperature changes or aging of the light sources 10 and 34and the photo-transistor and changes in ambient light will be cancelledby the negative feedback signal. In addition, the drive amplifier 78 or80, the light sources 10 or 34 and 40, the photo-transistor 14, tunedamplifier 86 and the negative feedback circuit 82, 87, 88 in effect forman optical operational amplifier whose gain is determined by the valuesof the passive circuit elements including the emitter resistor of thedrive amplifier transistor 78 and the resistors 140, 142 and capacitor144 of the phase shifter 87 in the feedback path 88 for better overallgain stability.

Of course, the tuned amplifier and its associated tank circuit changethe square wave signal pulses produced by the photo-transistor 14 inresponse to the light pulses of light sources 10 and 34 into a sine wavevoltage which is amplified. This amplified sine wave is then peakdetected and stored in a peak averager and memory circuit 90. Twoseparate memories are employed for storing the rain measurement signal,respectively, and the fog measurement signal and they are selectivelyconnected by an electronic switch (not shown) to the output of suchcircuit. The analog output signal of the peak averager and memorycircuit 90 is transmitted through a buffer amplifier 92 to one input 93of a voltage comparator 94.

A clock pulse generator 96 producing clock pulses having a frequency ofapproximately 20 Hertz is connected at its output 97 to a start input ofa staircase voltage generator 98 in order to enable such staircasegenerator to start to produce a stair-step voltage which increases onestep for each output pulse of the oscillator 74 whose output is alsoconnected to the staircase generator at a step input terminal 100. Thestair-step voltage generated at output terminal 102 of the staircasegenerator is connected to a second input of the voltage comparator 94 sothat when such stair-step voltage exceeds the averaged peak measurementanalog voltage at the first input 93 of the comparator such comparatorswitches to produce an output pulse at comparator output 104.

As stated the start input terminal of the gate 106 is connected to thestart output 97 of the clock 96 which starts the counter gate and thestaircase generator at same time. The output pulse of the comparator 94is fed to the stop input terminal of a counter gate 106 to turn off suchgate. As a result, counter gate 106 transmits output pulses of theoscillator 74 through such gate to the counter 108 for counting suchoscillator pulses to produce a digital output measurement signal at theoutput 110 of such counter which corresponds to the measurement of thedetected amount of rain or fog which has accumulated on the windshieldof the automobile. This digital measurement output signal at output 110is connected through a computer interface circuit 112 to a conventionaldigital computer, such as a microprocessor which uses the measurementvalue to control the operation of moisture removal devices. A resultready signal is applied by the output terminal 104 of the comparator 94to the computer interface 112 to enable it to process the digitalmeasurement signal produced at the output 110 of the counter.

Alternatively, for moisture measurement in a non-automobile applicationthe digital output signal of the counter may be transmitted from output110 to a display segment decoder circuit 116 which decodes the digitalsignal and applies a corresponding measurement signal to a three digitdisplay circuit 118 which displays the value of the moisturemeasurement. The clock 96 produces a reset signal which is applied tothe counter 108 to reset the counter to zero at the end of a measurementand a blanking signal to the display segment decoder 116 to blank suchdecoder between measurements.

It should be noted that the water accumulation measurement signal at theoutput of the counter 108 is a measure of both the number and size ofthe water droplets detected by the first light source 10 and thephoto-transistor 14 and therefore represents the total amount of wateraccumulated on the outer surface of the windshield. Also the value ofthis measurement signal increases with an increasing amount of waterdroplets on the outer surface of such windshield. However, when fog ismeasured on the inner surface of the windshield by the second lightsource 34 and the photo-transistor 14 the output signal of the counter108 decreases with increasing amount of fog. This difference between therain and fog signals is taken into account when the signals areprocessed by the computer 114 for a proper display of the measurementvalues of rain and fog and proper operation of moisture removal devicesby control signals at the control outputs 120 of the computer.

Also the electronic switch 76 for switching the output of the oscillator74 to either the input 75 of the driver amplifier 78 of the first fightsource 10 or the input 77 of the driver amplifier 80 of the second lightsource 34, is controlled by a control signal generated by a separatecontrol logic circuit or by the computer at one of the control outputs120 for alternately taking measurements of the rain droplet accumulationon the outer surface of the windshield or fog measurements of the amountof fog accumulation on the inner surface of the windshield. The computeroutput control signal is employed to operate various visibilityimproving devices such as windshield wipers which may be turned on andwhose speed may be varied by the computer depending upon the raindropaccumulation. Also electrical heaters and air blowers may be operated toremove the fog from the inner surface of the windshield of theautomobile. In addition, the computer output control signal can also beused to control motors for closing windows such as the sunroof window ofan automobile or raising the convertible top of a convertible-typeautomobile.

As shown in FIG. 7, the tuned narrowband amplifier circuit 86 has twoamplifier stages including a first stage having a first LC tank circuit122 including a first transformer 124 with its primary winding connectedin series with the collector of the photo-detector transistor 14 andhaving its secondary winding connected in parallel with a capacitor 126of the proper value so that such tank circuit is tuned to the 37Kilohertz frequency of the oscillator 74. It should be noted that aswitching transistor 127 is connected to the upper end of the primarywinding of transformer 124 to prevent the photo-transistor fromproducing a measurement signal when such switching transistor isswitched on to produce a hold signal which disables the measurementclock 96, such as when a high brightness ambient light drives thephoto-transistor into saturation. The output of the tank circuit 122 isconnected to the positive input of a first stage amplifier 128 through acoupling resistor 129. The oscillator through the electronic switch 76selectively applies the oscillator pulses to inputs 75 or 77 of thedriver amplifiers 78 or 80 for the first and second light emittingdiodes 10, or 34, respectively. It should be noted that the input 77 isconnected through a variable resistance potentiometer 79 to the emitterof the driver amplifier transistor 80 in order to adjust the amplitudeof the fog drive input signal, and the base of such transistor isconnected to the DC bias voltage at terminal 84. The DC bias voltagesources indicated as "+5d" and "+10d" are LC decoupled DC voltagesources of +5 volts and +10 volts.

The second stage of the tuned amplifier 86 includes a second tankcircuit 130 with a second transformer 132 having its primary windingconnected in series with a load resistor 134 to the output of amplifier128. The secondary winding of transformer 132 is connected in parallelwith a capacitor 136 of the proper value to tune the second tank circuit130 to the same 37 Kilohertz frequency of the oscillator. The output ofthe second tank circuit is connected through a coupling resistor 137 tothe positive input of a second amplifier 138 which produces an outputsignal voltage at its output terminal 139.

A negative feedback circuit is connected from the output 139 of thesecond amplifier 138 through an RC phase shift circuit 87 including aninput coupling resistor 140 an output coupling resistor 142 and a shuntcapacitor 144 connected from a point between such resistors and ground.

The negative feedback signal is applied to the emitter of a transistor146 in the current supply circuit 82 which supplies bias current for thethird light source 40. The base of transistor 146 is connected to asource of DC bias voltage at terminal 84 which normally biases suchtransistor conducting to cause current to flow from the collector ofsuch transistor through the light emitting diode (LED) 40 to normallybias such LED on so that it emits the third light beam. This third lightbeam is directed onto the photo-transistor 14 in order to optically biassuch photo-transistor at an operating point on its characteristic curveof high sensitivity to infrared light. It should be noted that thedriver amplifiers 78, 80 of the first light source 10 and second lightsource 34 are normally biased off and are switched into an on conditionby the square wave oscillator signals applied to input terminal 75 and77 by the electronic switch 76 as shown in FIG. 6. Thus, the lightsources 10 and 34 are pulsed on and off by the oscillator signal squarewave pulses to produce a corresponding pulsed output signal on thecollector of the photo-transistor 14 which is then changed into a sinewave by the tuned tank circuits 122, 130.

The negative feedback signal from the third light emitting diode 40 iscoupled by photo-transistor 14 and the primary winding of transformer124 to stimulate tank circuit 122 in a manner which is 180° out of phasefrom the stimulation produced in tank circuit 122 by the input signalfrom the first or second light emitting diode 10 or 34. As a result, thetank circuit voltage is reduced to a small fraction of what it wouldotherwise be with no feedback signal applied. The effects of sensitivitychanges in the light emitting diodes or photo-transistor caused bytemperature changes, aging, power supply variations or changes inambient light are also reduced accordingly. Using tank circuit 122 asthe starting point, the voltage produced by the input signal from thefirst or second light emitting diode 10 or 34 is phase shifted a totalof 22° in the circuits associated with amplifier 128, transformer 132,tank circuit 130 and amplifier 138. Phase shifted circuit 87 adds 68°while transistor 82, light emitting diode 40 and photo-transistor 14 donot add appreciable phase shift. The normal phase difference atresonance between the inductor current and the capacitor voltage in tankcircuit 122 adds another 90° for a total phase shift around the loop of180°.

The sine wave output signal of the second amplifier stage 138 of thenarrowband amplifier 86 is transmitted from output terminal 139 to theinput of the peak averager and memory circuit 90 where it is averagedand stored as a DC analog voltage in either a rain memory capacitor 148or a fog memory capacitor 150. A first charging gate including a firstpair of anode connected gating diodes 152, 154 is connected between theoutput of amplifier 138 and the upper plate of rain memory capacitor 148to charge such capacitor to the peak amplitude of the rain measurementoutput signal only when such gate is rendered conducting by a computercontrol square wave gate signal applied to a gate terminal 156 connectedto the common connection of the anodes of such diodes. Switchingtransistor 158 is connected as a shunt to the +5 volts DC supply betweenrain memory capacitor 148 and the memory output 172. During a rainmeasurement, a square wave signal applied to control terminal 160connected to the gate of field effect transistor 158 renders itnon-conducting such that the rain measurement signal stored in rainmemory capacitor 148 reaches the memory output 172. During a fogmeasurement, however, transistor 158 is rendered conducting thusinhibiting the stored rain measurement signal from reaching the memoryoutput 172. A similar charge gate 162, 164 and switching transistor 168are provided for the fog memory capacitor 150. Thus, the fog memorycapacitor 158 is connected through a second charging gate formed by apair of diodes 162 and 164 having their common anode connectionconnected to a gate control input 166 for rendering such gate conductiveto charge the fog memory capacitor 150 from the output of the amplifier138 through such gate. Switching transistor 168 is connected as a shuntto the +5 volts DC supply between fog memory capacitor 150 and thememory output 172. During a fog measurement, a square wave signalapplied to control terminal 170 connected to the gate of field effecttransisor 168 renders it non-conducting such that the fog measurementsignal stored in fog memory capacitor 150 reaches the memory output 172.During a rain measurement, however, transistor 168 is renderedconducting thus inhibiting the stored fog measurement signal fromreaching the memory output 172. It should be noted that the chargingcontrol signals on terminals 156 and 166 are square waves which arephase inverted with respect to each other so that gate 152, 154 is openwhen gate 162, 164 is closed and vice versa. However, there is a timedelay between the termination of the gate on signal at terminal 156 andthe start of the gate on signal at terminal 166. During such time delaya charge voltage on the rain memory capacitor 148 is transmitted throughthe buffer amplifier 92 to the comparator for operating the counter gate106 to cause the counter 108 to count the rain measurement in FIG. 6.The rain measurement signal at counter output 110 is subsequentlydisplayed after the count is completed mad while the fog signal ischarging fog memory capacitor 150.

The disabling control signals on terminals 160 and 170 are phaseinverted with respect to each other so that switch 158 is on whileswitch 168 is off and vice versa. As a result, depending upon whetherswitches 158 and 168 are on or off either the rain measurement signalstored on memory capacitor 148 or the fog measurement signal stored onmemory capacitor 150 is supplied from the output 172 of the memorythrough the buffer amplifier 92 to the comparator 93 of FIG. 6. In thisway, the moisture detection system produces with light sources 10 and 34at different times the two output measurement signals at the output 110of the counter 108 including a rain measure signal corresponding to theraindrop accumulation on the outer surface of the windshield and a fogmeasurement signal corresponding to the fog accumulation on the innersurface of such windshield.

It should be noted that the charge control signals applied to controlterminals 156, 166 and the disabling control signals applied to controlterminals 160, 170 are all produced by the computer and supplied fromdifferent ones of its control output terminals 120 at appropriate timesas is the control signal for operating the electronic switch 76 forselecting light sources 10 and 34 which determines whether rain or fogmeasurements are to be taken.

It should be noted that the above-described preferred embodiments of thepresent invention are merely illustrative of the present invention. Manychanges may be made in such preferred embodiments which will be obviousto those having ordinary skill in the art. Therefore, the scope of thepresent invention should only be determined by the following claims.

I claim:
 1. An optical droplet detector apparatus for determining thedegree of vision impairment through a window due to an accumulation ofwater or other precipitation on the window comprising:a first lightsource for illuminating water droplets on said window with a first lightbeam transmitted through said window to the inside of said window; aphoto-detector located inside said window; and a masking devicepositioned in front of said photo-detector to effectively block thedirect transmission of light rays from said first light source to saidphoto-detector, but allowing said photo-detector to receive light raysfrom said first source which have been refracted by droplets on saidwindow for the purpose of determining the amount of water on said windowso that the output signal of said photo-detector increases with anincrease in water droplets on said window.
 2. Apparatus in accordancewith claim 1 wherein the first light beam detects rain droplets on theoutside surface of the window, the masking device is positioned betweenthe first light source and the photo-detector and which also includes asecond light source positioned inside said window to produce a secondlight beam for detecting fog on the inside surface of the window whichreflects off the inside surface to said photo-detector for the purposeof determining the degree of fog accumulation on said inside surface,said first and second light sources emitting infrared light. 3.Apparatus in accordance with claim 1 which also includes a third lightsource positioned inside said window with its beam directly aimed atsaid photo-detector for the purpose of optically driving saidphoto-detector to its bias operating point and for providing an opticalfeedback signal to said photo-detector.
 4. Apparatus in accordance withclaim 3, further comprising a drive circuit for providing pulsed drivecurrent to said first light source for water accumulation measurementsand another drive circuit connected to said third light source for gainstabilization, and an output circuit for amplifying the pulsating outputcurrent produced by the photo-detector.
 5. Apparatus in accordance withclaim 4, wherein said output circuit includes a narrowband amplifier andtank circuit closely tuned to the frequency of the pulsed drive currentfor the purpose of rejecting other, potentially interfering currentsproduced by said photo-detector.
 6. Apparatus in accordance with claim4, wherein a feedback circuit is connected from said output circuit tothe drive circuit for the third light source to provide an out-of-phase,negative feedback signal to modulate said third light source to providean optical operational amplifier circuit for the purpose of overall gainstabilization.
 7. Apparatus in accordance with claim 1, furthercomprising detector circuit means connected to the output of thephoto-detector for peak detection, averaging and temporary storage ofthe photo-detector output signals to produce output signal datapertaining to visibility impairment of said window, reducing the effectsof noise and unwanted signals in the data, and temporarily storing thedata for subsequent analog-to-digital conversion or thresholdcomparison.
 8. Apparatus in accordance with claim 2, further comprisingan analog-to-digital converter for converting the analog output signalof the photo-detector pertaining to visibility impairment to a pair ofdigital output signal related to measurements for rain and fog,respectively, and computer interface circuits to interface said digitaloutput signal to a microprocessor for the purpose of making thresholdand state-transition decisions to control devices for restoringvisibility or for closing windows.
 9. Apparatus in accordance with claim8, wherein a digital offset measurement is made of the output signals ata zero-drive-signal condition obtained when neither said first lightsource nor said second light source are driven for the purpose ofsubtracting said offset value from the output signals corresponding toeach subsequent rain and fog measurement to eliminate any residualzero-offset errors from said rain and fog measurements.
 10. A waterdroplet detector for determining the degree of vision impairment througha windshield of a vehicle due to an accumulation of water on saidwindshield comprising:a first sensor incorporating a first light sourceand a photo-detector for the purpose of determining the degree of wateraccumulation on the outside surface of said windshield; a second sensorincorporating a second light source and said photo-detector for thepurpose of determining the degree of fog accumulation on the insidesurface of said windshield; a third light source for the purpose ofoptically driving said photo-detector to its bias operating point; adrive circuit for providing pulsed drive current to said first lightsource and to said second light source; an output circuit connected tothe photo-detector including a tuned amplifier for providing narrowbandamplification closely tuned to the frequency of said pulsating drivecurrents for the purposes of amplifying the pulsating output currentproduced by said photo-detector and rejecting other, potentiallyinterfering currents produced by said photo-detector; and a feedbackcircuit connected from said output circuit to said third light sourcefor providing attenuation and additional phase shift to the outputsignal of said output circuit to provide a negative feedback signal tomodulate said third light source for the purpose of gain stabilizationof said output circuit.
 11. Apparatus in accordance with claim 10,further comprising a detector circuit means connected to the outputcircuit for peak detection, averaging and temporary storage of theoutput signals for producing output signal data pertaining to visibilityimpairment of said windshield, for reducing the effects of noise andunwanted signals in the data, and for temporarily storing the data forsubsequent analog-to-digital conversion or threshold comparison. 12.Apparatus in accordance with claim 10, further comprising ananalog-to-digital converter for converting the analog output signals ofthe output circuit pertaining to visibility impairment to a pair ofdigital output signals relating to measurements for rain and fog,respectively, computer interface circuits to interface said digitaloutput signals to a microprocessor for the purpose of masking thresholdand state-transition decisions to control devices for restoringvisibility or for closing windows, and offset means for measuringdigital offset of the output signals at a zero-drive-signal conditionobtained when neither said first light source nor said second lightsource are driven, for the purpose of subtracting said offset value fromthe output signals corresponding to each subsequent rain and fogmeasurement to eliminate any residual zero-offset errors from said rainand fog measurements.
 13. Apparatus in accordance with claim 10 in whichthe light sources and their drive circuits together with thephoto-detector and its output circuit and feedback circuit are connectedas an optical operational amplifier for gain stability.
 14. A method ofoptical detection of water or other precipitation on a window todetermine the degree of vision impairment through the window due to theaccumulation of water on said window, comprising the stepsof:transmitting a first light beam from a first light source throughsaid window to the inside of said window; detecting the first light beamwith a photo-detector inside said window; blocking the directtransmission of light rays from said first light source to saidphoto-detector by a light masking device; and refracting said firstlight beam with water droplets or other precipitation on said window tocause a portion of said first light beam to be redirected from saidwater droplets to said photo-detector so that the detector output signalof said photo-detector increases with an increase in water droplets onsaid window.
 15. A method in accordance with claim 14 in which the firstlight beam is transmitted from a first light source outside the windowand is refracted by rain droplets on the outside surface of the window,and which also includes the step of:transmitting a second light beamfrom a second light source inside the window so that said second lightbeam is reflected off the inside surface of the window to thephoto-detector for determining the amount of fog accumulation on saidinside surface by decreases in the detector output signal with increasesin fog on the inner surface of said window due to diffusion of the lightbeam by the fog.
 16. A method in accordance with claim 14 which alsoincludes the step of:transmitting a third light beam from a third lightsource directly to the photo-detector to optically drive thephoto-detector to its bias operating point.
 17. A method in accordancewith claim 15 which also includes the step of:applying pulsed drivecurrent selectively to the first light source and to the second lightsource to cause them to emit light which is pulsed at a predeterminedfrequency to cause the photo-detector to produce a pulsed output signal.18. A method in accordance with claim 17 which also includes the stepof:transmitting the pulsed output signal of the photo-detector throughan output circuit including a narrowband amplifier and tank circuittuned to the frequency of the pulsed drive current.
 19. A method inaccordance with claim 18 which also includes the step of:transmitting anegative feedback signal from the photo-detector output circuit to thedrive circuit of the third light source.
 20. A method in accordance withclaim 18 which also includes the steps of:peak detection of the analogphoto-detector output signal at the output of the photo-detector outputcircuit; converting the analog photo-detector output signal to a digitaloutput signal; and processing said digital output signal with a computerto produce digital data corresponding to measurements of the amount ofwater on the window. .Iadd.
 21. An optical droplet detector apparatusfor determining the degree of vision impairment through a window due toan accumulation of water or other precipitation on the windowcomprising:a first light source for illuminating water droplets on saidwindow with a first light beam to refract a portion of said first lightbeam by said droplets; a photo-detector located inside said window; anda masking device positioned in front of said photo-detector toeffectively block the direct transmission of light rays from said firstlight source to said photo-detector, but allowing said photo-detector toreceive light rays from said first source which have been refracted bydroplets on said window for the purpose of determining the amount ofwater on said window so that the output signal of said photo-detectorincreases with an increase in water droplets on said window..Iaddend..Iadd.
 22. Apparatus in accordance with claim 21 wherein thefirst light beam detects rain droplets on the outside surface of thewindow, the masking device is positioned between the first light sourceand the photo-detector and which also includes a second light sourcepositioned inside said window to produce a second light beam fordetecting fog on the inside surface of the window which reflects off theinside surface of the window to said photo-detector for the purpose ofdetermining the degree of fog accumulation on said inside surface, saidfirst and second light sources emitting infrared light. .Iaddend..Iadd.23. Apparatus in accordance with claim 21 which also includes a thirdlight source positioned inside said window with its beam directlytransmitted to said photo-detector for the purpose of optically drivingsaid photo-detector to its bias operating point and for providing anoptical feedback signal to said photo-detector;a drive circuit forproviding pulsed drive current to said first light source for wateraccumulation measurements and another drive circuit connected to saidthird light source for gain stabilization, and an output circuit foramplifying the pulsating output current produced by the photo-detector;said output circuit includes a narrowband amplifier for the purpose ofrejecting other, potentially interfering currents produced by saidphoto-detector. .Iaddend..Iadd.24. Apparatus in accordance with claim 23wherein a feedback circuit is connected from said output circuit to thedrive circuit for the third light source to provide a negative feedbacksignal to modulate said third light source to provide an opticallycoupled operational amplifier circuit for the purpose of overall gainstabilization. .Iaddend..Iadd.25. A water droplet detector fordetermining the degree of vision impairment through a windshield of avehicle due to an accumulation of water on said windshield comprising:afirst sensor incorporating a first light source and a photo-detector forthe purpose of determining the degree of water accumulation on theoutside surface of said windshield; a reference light source for thepurpose of optically driving said photo-detector to its bias operatingpoint; a drive circuit for providing pulsed drive current to said firstlight source; an output circuit connected to the photo-detectorincluding an amplifier for the purpose of amplifying the pulsatingoutput current produced by said photo-detector and rejecting other,potentially interfering currents produced by said photo-detector; and afeedback circuit connected from the output of said output circuit tosaid reference light source to provide a negative feedback signal tomodulate said reference light source which optically couples saidfeedback signal to said photo-detector for the purpose of gainstabilization of said output circuit. .Iaddend..Iadd.26. Apparatus inaccordance with claim 25, further comprising an analog-to-digitalconverter for converting the analog output signals of the output circuitpertaining to visibility impairment to digital output signals relatingto measurements, computer interface circuits to interface said digitaloutput signals to a microprocessor for the purpose of making thresholdand state-transition decisions to control devices for restoringvisibility or for closing windows, and offset means for measuring thedigital offset of the output signals at a zero-drive-signal conditionfor the purpose of subtracting said offset value from the output signalcorresponding to each subsequent measurement to eliminate any residualzero-offset errors from said measurements. .Iaddend..Iadd.27. Apparatusin accordance with claim 25 in which the light sources and their drivecircuits together with the photo-detector and its output circuit andfeedback circuit are connected as an optically coupled operationalamplifier for gain stability. .Iaddend..Iadd.28. A method of opticaldetection of water or other precipitation on a window to determine thedegree of vision impairment through the window due to the accumulationof water on said window, comprising the steps of: transmitting a firstlight beam from a first light source to illuminate water droplets onsaid window; detecting the first light beam with a photo-detector insidesaid window; blocking the direct transmission of light rays from saidfirst light source to said photo-detector by a light masking device; andrefracting said first light beam with water droplets or otherprecipitation on said window to cause a portion of said first light beamto be redirected from said water droplets to said photo-detector so thatthe detector output signal of said photo-detector increases with anincrease in water droplets on said window. .Iaddend..Iadd.29. A methodin accordance with claim 28 in which the first light beam is transmittedfrom a first light source and is refracted by rain droplets on theoutside surface of the window, and which also includes the stepof:transmitting a second light beam from a second light source insidethe window so that said second light beam is reflected off the insidesurface of the window to the photo-detector for determining the amountof fog accumulation on said inside surface by decreases in the detectoroutput signal with increases in fog on the inner surface of said windowdue to diffusion of the light beam by the fog. .Iaddend..Iadd.30. Amethod in accordance with claim 29 which also includes the stepsof:applying pulsed drive current selectively to the first light sourceand to the second light source to cause them to emit light which ispulsed at a predetermined frequency to cause the photo-detector toproduce a pulsed output signal; and transmitting the pulsed outputsignal of the photo-detector through an output circuit including anarrowband amplifier. .Iaddend..Iadd.31. An optical droplet detectorapparatus for detecting the presence of water droplets on the windshieldof a motor vehicle, comprising: a first light source for illuminatingwater droplets on the outer surface of the windshield with a first lightbeam to refract a portion of said first light beam by said droplets; aphoto-detector located inside the vehicle; and a masking devicepositioned to prevent the direct transmission of light from said firstlight source to said photo-detector to cause the photo-detector toreceive light from said first light beam when said first light beam isrefracted by water droplets on said windshield so that the output signalof said photo-detector increases in amplitude with an increase in theamount of water droplets on said windshield. .Iaddend..Iadd.32.Apparatus in accordance with claim 31 which also includes a referencelight source for producing a reference light beam which is directlytransmitted to the photo-detector to bias its operating point..Iaddend..Iadd.33. Apparatus in accordance with claim 32 in which thephoto-detector is connected to the input of a detector circuit which hasa negative feedback circuit connected from the output of said detectorcircuit to said reference light source to provide an optical feedbacksignal to said photo-detector. .Iaddend..Iadd.34. Apparatus inaccordance with claim 33 in which the first light source and thereference light source are connected to drive circuits for producingpulsed drive current which pulses both light sources on and off..Iaddend..Iadd.35. Apparatus in accordance with claim 31 which alsoincludes a second light source positioned inside the vehicle forproducing a second light beam which is transmitted to the inner surfaceof the windshield where it is redirected to the photo-detector by fog onsaid inner surface in order to detect said fog. .Iaddend.